1. Field
The present disclosure relates to a method and a system for detecting gloss non-uniformities on printed documents.
2. Description of Related Art
An electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
Many documents produced by such process, and especially, color image printing process, have a need for a uniform, high gloss. Gloss is the property of a substrate surface which involves specular reflection. Specular reflection is a sharply defined light beam resulting from reflection off a smooth, uniform surface. Gloss follows the law of reflection which states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection. Gloss properties are generally measured in Gardner Gloss Units (ggu). Gloss varies due to various factors, including paper properties, toner properties and toner mass per unit area (TMA), oil levels, fuser roll age, and temperature variability.
Image quality defects in gloss are common across different image printing systems. For example, such image quality defects in gloss may include transfer roller ghosting (e.g., in solid inkjet printing), edge wear streaks, etc.
The transfer roller ghosting is caused by a previous impression left behind a residual imprint of oil on a transfer roller that is subsequently imparted on the finish surface of a subsequent print while being transfixed.
Many fuser issues may also lead to non-uniform gloss or a gradual change in gloss as the fuser ages. Examples of fuser roll induced gloss defects include edge wear streaks, oil streaks, and air knife streaks such as from cool rings (due to non-uniform cooling of the fuser roll or recording medium after fusing) and random wavy gloss and gloss reduction as material accumulates on the roll that contacts the image. The edge wear streaks in gloss, for example, are caused by a worn fuser roller of the image printing system. Post fuser temperature disparities also alter the image gloss. Post fuser gloss defects include belt hole artifacts from the post fuser belt transport.
Document scanners are typically used in image quality measurement methods to capture scanned images of the prints being measured. For example, these image quality measurement methods are developed to quantify print quality defects such as uniformity (streaks and bands), mottle & graininess, line and text quality, color quality and so on. However, document scanners typically are designed to be insensitive to imaging of gloss and as such they are not capable of detection of image quality non-uniformities or defects in gloss.